Relationship between the NO<sub>2</sub> photolysis frequency and the solar global irradiance

Abstract

Abstract. Representative values of the atmospheric NO 2 photolysis frequency j (NO 2 ) are required for the adequate calculation and interpretation of NO and NO 2 concentrations and exchange fluxes near the surface. Direct measurements of j (NO 2 ) at ground level are often not available in field studies. In most cases, modeling approaches involving complex radiative transfer calculations are used to estimate j (NO 2 ) and other photolysis frequencies for air chemistry studies. However, important input parameters for accurate modeling are often missing, most importantly with regard to the radiative effects of clouds. On the other hand, solar global irradiance ("global radiation", G) is nowadays measured as a standard parameter in most field experiments and in many meteorological observation networks around the world. Previous studies mainly reported linear relationships between j (NO 2 ) and G. We have measured j (NO 2 ) using spectro-or filter radiometers and G using pyranometers side-by-side at several field sites. Our results cover a solar zenith angle range of 0-90 • , and are based on nine field campaigns in temperate, subtropical and tropical environments during the period 1994-2008. We show that a second-order polynomial function (intercept = 0): and B 2 = (−4.84 ± 0.31) × 10 −9 W −2 m 4 s −1 can be used to estimate ground-level j (NO 2 ) directly from G, independent of solar zenith angle under all atmospheric conditions. The absolute j (NO 2 ) residual of the empirical function is ±6 × 10 −4 s −1 (2σ ). The relationship is valid for sites below 800 m a.s.l. and with low surface albedo (α < 0.2). It is not valid in high mountains, above snow or ice and sandy or dry soil surfaces.

Quantification of the unknown HONO daytime source and its relation to NO&lt;sub&gt;2&lt;/sub&gt;

“…NO 2 photolysis frequencies j (NO 2 ) were measured by filter radiometers (Meteorologie consult, Königstein, Germany) on top of the scaffold, with an uncertainty of ±5 %. The HONO photolysis frequency (j (HONO)) was calculated by multiplying j (NO 2 ) with a factor of 0.175 (Trebs et al, 2009). By comparing different parameterizations (Kraus and Hofzumahaus, 1998;Trebs et al, 2009), the uncertainty for the calculation of j (HONO) was estimated to be 5 %.…”

“…The HONO photolysis frequency (j (HONO)) was calculated by multiplying j (NO 2 ) with a factor of 0.175 (Trebs et al, 2009). By comparing different parameterizations (Kraus and Hofzumahaus, 1998;Trebs et al, 2009), the uncertainty for the calculation of j (HONO) was estimated to be 5 %. The overall (4π ) photolysis frequency was calculated by increasing values of the downwelling radiation by 5 %, i.e.…”

NO&lt;sub&gt;2&lt;/sub&gt; photolysis frequencies in street canyons

“…In general, for simplification with respect to the short computational time that is available in photochemical models, the photolysis frequencies are given with simple parameterizations (Roeth, 2002;Trebs et al, 2009). Thus, in the present paper, ratios of the photolysis frequency in the street and its value for the same atmospheric conditions, but with free horizon, are derived.…”

“…Parameterized photolysis frequencies (Röth, 2002) are available only for undisturbed conditions with free horizon, since they are based on values of the global solar irradiance G, measured for such conditions (Trebs et al, 2009).…”

Effects of temperature-dependent NO&lt;sub&gt;x&lt;/sub&gt; emissions on continental ozone production

“…The NO 2 photolysis rate was parameterized as a quadratic function of total solar radiation (Trebs et al, 2009). Using this method and scaling the result to match the values calculated using steady-state RO 2 concentrations during SOAS, we find that (Fig.…”